Photonuclear reactions triggered by lightning discharge
Enoto, Teruaki ; Wada, Yuuki ; Furuta, Yoshihiro ; Nakazawa, Kazuhiro ; Yuasa, Takayuki ; Okuda, Kazufumi ; Makishima, Kazuo ; Sato, Mitsuteru ; Sato, Yousuke ; Nakano, Toshio ; Umemoto, Daigo ; Tsuchiya, Harufumi
Lightning and thunderclouds are natural particle accelerators. Avalanches of relativistic runaway electrons, which develop in electric fields within thunderclouds, emit bremsstrahlung ?-rays. These ?-rays have been detected by ground-based observatories, by airborne detectors and as terrestrial ?-ray flashes from space. The energy of the ?-rays is sufficiently high that they can trigger atmospheric photonuclear reactions that produce neutrons and eventually positrons via ?+ decay of the unstable radioactive isotopes, most notably 13N, which is generated via 14N?+?????13N?+?n, where ? denotes a photon and n a neutron. However, this reaction has hitherto not been observed conclusively, despite increasing observational evidence of neutrons and positrons that are presumably derived from such reactions. Here we report ground-based observations of neutron and positron signals after lightning. During a thunderstorm on 6 February 2017 in Japan, a ?-ray flash with a duration of less than one millisecond was detected at our monitoring sites 0.5-1.7 kilometres away from the lightning. The subsequent ?-ray afterglow subsided quickly, with an exponential decay constant of 40-60?milliseconds, and was followed by prolonged line emission at about 0.511 megaelectronvolts, which lasted for a minute. The observed decay timescale and spectral cutoff at about 10 megaelectronvolts of the ?-ray afterglow are well explained by de-excitation ?-rays from nuclei excited by neutron capture. The centre energy of the prolonged line emission corresponds to electron-positron annihilation, providing conclusive evidence of positrons being produced after the lightning.
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